This invention relates generally to telecommunications and more particularly to line drivers that are used in telecommunication equipment.
As is known, communication systems include a plurality of users (e.g., telephones, personal computers, facsimile machines, web information resources, et cetera) that communicate data (e.g., audio, voice, video, text, and/or multi-media) via a communication network. The communication network, which may be a public switch telephone network (PSTN), the Internet, local area network (LAN), wide area network (WAN), et cetera, includes a plurality of network switches. To facilitate the conveyance of data, the communication network employs one or more standardized data transmission protocols, such as asynchronous transfer mode (ATM), asymmetrical digital subscriber line (ADSL), integrated services digital network (ISDN), et cetera.
Regardless of the data transmission protocol, the transmission of data between user and network switch, network switch and network switch, and network switch and user is via a wireless, wireline, and/or fiber optic transmission line. As is known, high frequency components of signals that are transmitted over a wireless transmission are adversely effect by the frequency response characteristics of the wireline transmission line. To reduce the adverse effects, each of the data transmission protocols includes techniques to recover the high frequency components of the transmitted data and requires the use of line drivers.
As is known, line drivers receive a low power input signal and increase its power such that the signal can drive a wireline transmission line. In essence, the line drivers are power amplifiers that accurately reproduce the input signal (i.e., with negligible distortion) and may include conventional architectures, such as Class A amplifier, Class B amplifier, Class AB amplifier, etc. While conventional line drivers work well for many applications, they become less and less effective as the supply voltage decreases and the data rates increase. For example, at a 1.6 volt supply voltage and data rates in excess of 200 Mbps (megabits per second), conventional line drivers introduce non-negligible distortion to the signal, making them unacceptable for such applications. The distortion is introduced by the parasitic properties of the components used to implement the line driver.
In addition to the above issues for conventional line drivers, they typically require a gain stage to amplify the magnitude of an incoming signal before an acceptable drive signal can be produced when operated at low voltages. Gain stages are typically implemented using an operational amplifier and resistors interoperably coupled to produce the desired gain. For high-speed data rates (e.g., in excess of 200 Mbps), the parasitic properties of the operational amplifier distort the high-speed data, rendering conventional gain stages an impractical pre-amplifier for high-speed line drivers. Alternatively, a switched capacitor circuit may provide the gain stage, but for high-speed data rates, the switched capacitor circuit would require a very fast and accurate clock. While such a clock may be implemented with today""s technology, it adds additional cost to the line driver. When one considers that hundreds of millions of line drivers are fabricated each year for various types of telecommunication equipment, even a fraction of a penny of additional cost due to a very fast and accurate clock adds up to millions of dollars per year. Another drawback of switched capacitor circuits is that they introduce noise into the substrate, thereby adversely affecting other circuits on the substrate.
Therefore, a need exists for a high-speed line driver that includes gain, introduces negligible distortion, has good resolution, adjustable output impedance to match the line impedance, and can be economically fabricated.